DC coupled 2.7 GHz Active Probe Project - Now Available!

[lasmux]

UPDATE, 17/04/2024: These are now available at http://www.lasmux.com/product/single-ended-active-probes/. 2.7GHz probe.

I've been working on an active probe design for around a year. The goal started off as creating a DC-coupled active probe to support a photon counting sensor I am also working on, but it was a very fun project and I spent so much time on it that now the plan is to sell it. Could I have some feedback on the probe/performance, and on the contents of the datasheet, before I start buying the first batch of parts... which will be quite expensive.

I have another post that I'm putting together where I'll go into the development process a bit more.

I'm making two versions, a 1GHz version, and a 2GHz version.

The datasheet is here: https://www.lasmux.com/wp-content/uploads/2023/07/LD-ASP-1G_2G.pdf



Quick specs:
Bandwidth: DC-1GHz, DC-2GHz
Input capacitance (measured at 1GHz): 0.7pF
Attenuation: 20x
DC input resistance: 1Mohm

1GHz version frequency response (linear and log axis):


2GHz version frequency response (linear and log axis):


Tip input impedance of both probes, depending on which ground lead is fitted:


The resistive ground lead can be used to stop a resonance developing on the ground connection, which reduced the input impedance above 1.5GHz. I talk about this a bit more in the datasheet.



In terms of step response for the system, I've 'only' got a 500MHz oscilloscope, so can't properly test the rising edge speed unfortunately. This is the probe measuring a 50 ohm terminated 100MHz signal, with a <100ps rise time. This greatly exceeds the bandwidth of the scope so there's some ringing. The trace looks basically identical if I measure the signal directly by the oscilloscope.

Currently I'm aiming for around £150 for the 1GHz version, and £185 for the 2GHz version.

[lasmux]

There have been... quite a lot of iterations of the hardware design...



I started off with a BNC output on the probe itself, and the batteries mounted to the probe also, but then went for an SMA connector and an external 9V battery box. It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

I went through a lot of iterations on the amplifier and signal conditioning circuitry, and got very used to soldering 0402's, which made my life miserable. Also given the extremely small capacitances on the board, every single time I made a change to the passive networks, I'd have to clean the board in IPA to remove flux residues.

[djsb]

Hi,
When will this be back in stock? Or is it initially for evaluation purposes only (as you mention hardware changes)? Thanks.

[lasmux]

I've not made the first batch yet. I'd expect maybe six to eight weeks, assuming my PCB assembler doesn't mess me about.

If the response from this thread is positive, I'll make an initial batch of 25 and start the process immediately.

[lasmux]

Here is a simplified schematic of the probe input to give you a gist of how it works.



C1 balances the parasitic capacitance across R2, to get a resistor/capacitor divider between R2/R3. As you could imagine, the parasitic capacitance across R2 is extremely low, given it's an 0402 passive. By varying C1 and observing the probe frequency response, I measured it at less than 0.1pF! This is not a lot of capacitance.

The capacitance of the probe tip is a lot more than 0.1pF (0.7pF) because there is some coupling between the probe tip and ground itself. This is not so difficult to avoid by just isolating that resistor from ground completely. Unfortunately, the more you pull ground away from R2, the worse the noise pick-up on the probe is. The net which connects R2 to the non-inverting amp input is high impedance, and if you pull the ground away from it, it picks up a lot of noise. One of my design iterations removed a lot of the ground planes around this net to really try to keep the tip capacitance down as low as possible, and the noise pickup was really bad. So you kinda have to choose your demon. The noise level is quite a lot better than other active probes (only 650uV), but because it's a x20 probe, I needed to keep the noise level to a minimum.

One thing which I could do in future is reduce the R2 to 200k or something, so I would have to correspondingly reduce R3, which would make that node much lower impedance and less succeptible to noise. I could then pull the ground a bit further back to try to further reduce the tip capacitance. Not sure.

The 1Mohm resistor R2 also protects the non-inverting input of the high speed FET input op amp from ESD damage. It does have ESD protection diodes built in, but the resistor helps keep them safe. A lot safer than just having discrete FETs on the inputs.

[nctnico]

It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

You can buy these cables off-the-shelve from China.

[lasmux]

It does mean more cables, and the coax cable needs to be custom to go from SMA-BNC, but it is a much easier device to use because of it.

You can buy these off-the-shelve from China.

lol. I've just looked and they're everywhere on ebay etc. I have no idea how I missed that  . Thanks.

[Weston]

Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.

Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experience https://www.crowdsupply.com/weston-braun/little-bee

Is the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

[lasmux]

Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.

It's not injection moulded, it's MJF 3D printed, and then vibro polished to get a smooth finish. Really quite remarkable how comparable it is to injection moulding. I should do a post on the mechanical side of things also.

Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experience.

I'd not known about this. Very cool that they distribute through Mouser. I'll have to look into this. Did you have to promote the crowd funding event? Getting 30k in funding is very impressive! I wonder if they'd accept a self certification CE process.

Is the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

Yes it works with AC coupling on the input or output. So long as the instrument it's plugged into can operate with small DC offsets on the input, as obviously the output is not AC coupled. My VNA didn't have any trouble.

[lasmux]

Modelling was completed in the venerable Fusion 360. I'm not the best at mechanical design so this was quite a learning curve to create an ergonomic (ish) design.



I then went through a number of different 3D printing technologies till I found one I was happy with. The PCB is a very close fit with the enclosure so I needed really good dimensional accuracy, and I didn't want lots of layer lines visible either, which would be more apparent on such a small part.


From left to right we have:
MJF nylon 12, vibro-polished. This was the best result which combined a smooth finish with excellent dimensional accuracy.
SLS nylon 12. Good enough dimensional accuracy, but the finish wasn't smooth enough. I tried melting/smoothing the surface by passing a hot air gun over it lightly. This did work, and ended up with a nice gloss, but was quite risky as a little too much heat and the thin walls of the part would start to distort.
SLA resin "tough 2k". Blotchy finish and not enough dimensional accuracy. Also the support structure was more apparent where it was broken off.
SLS nylon 12. In grey, same issues as before.
SLA resin "rigid 10k". A glass filled resin which gives a really smooth finish, unfortunately the dimensional accuracy wasn't quite good enough. The support structures here weren't as visible.
SLA resin "clear". Blotchy finish, and not enough dimensional accuracy. Had quite a lot of issues with the support structures not having broken off cleanly with this one.

[KungFuJosh]

I want one. I don't ever measure anything that I really need an active probe for, but that looks cool. 😉

[lasmux]

Thanks

It's quite good as a general purpose high-speed probe, of if you want to measure a moderately fast or high impedance signal... but yes, it's not for everyone.

I was also wondering if it could be used as a sensor preamp, perhaps for a high-speed photodiode or something.

[mawyatt]

Really cool work! It looks like the plastic for the probe tip is injection molded? Thats a lot of commitment.

Have you considered selling this through crowd supply? There might be a bit of product testing required (CE certification), but Crowd Supply provides a direct path to distribution though Mouser. It's also a lot easier to not have to deal with fulfillment yourself and the crowdfunding model removes any monetary risks with fronting money for parts. Its now discontinued, but I sold the Little Bee current probe through Crowd Supply and it was a great experience https://www.crowdsupply.com/weston-braun/little-bee

Is the probe stable with an AC coupled 50 ohm termination? A lot of equipment, such as VNAs/Spectrum Analyzers have an AC coupled 50 ohm termination. You obviously don't need a probe that works down to DC for that, but this would be a cost effective high impedance probe.

We have the Little Bee B1, don't use it much, but when we need to measure smaller currents it's served well 

Thanks for the product

Best,

[mawyatt]

@ lasmux,

Nice project, well done

We didn't know about the MJF printing, that case looks very nice indeed!!

You certainly picked up on the Fusion 360, and produced a nice rendering as well!!

A question, why the selection of 26dB rather than 20dB attenuation?? Guessing limits of the +-2.5V supply rails?

Best,

[nctnico]

1:10 for the probe and another 1:2 for the 50 Ohm termination makes 26 dB.

[lasmux]

@ lasmux,
Nice project, well done
We didn't know about the MJF printing, that case looks very nice indeed!!
You certainly picked up on the Fusion 360, and produced a nice rendering as well!!
A question, why the selection of 26dB rather than 20dB attenuation?? Guessing limits of the +-2.5V supply rails?

Thanks for the encouraging words

Yes, MJF 3D printing is amazing. Combined with vibro-polishing I feel it sets a new standard for low-volume projects.

I could have made it x10 into the 50 ohm, but I would have had to double the gain of the op amp which would have reduced the bandwidth somewhat. Also it would have reduced the measurable voltage range to +/-7V due to the limited rails, as you alluded to.

[lasmux]

To add. I also couldn't reduce the input voltage divider as it would have also meant reducing C1 by half (see schematic above) to maintain the capacitor divider ratio. C1 is a combination of an 0402 capacitor, and parasitic capacitance to ground. The 0402 capacitor part of C1 would have likely needed to be a smaller value than the smallest 0402 capacitor available.

[lasmux]

Yes, theoretically, it's a 10:1 probe if you put it into a 1Mohm oscilloscope, but then with a 1 metre coax cable, you're going to get reflection artefacts causing issues over 20MHz or so. Typically for 50 ohm terminated active probes, they say the attenuation as measured across a 50 ohm terminated scope.

[JohnG]

This looks like a really nice and affordable probe.

I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.

Thanks,
John

[lasmux]

This looks like a really nice and affordable probe.
I have a question regarding large signal performance. The datasheet alludes to something that sounds like slew rate limiting that may affect the large-signal bandwidth. This is important to me because my primary application would be a scope probe. Have you done anything to look as this, or can you find someone with a high-bandwidth scope and source to look at it? I'm happy to volunteer, but I bet there are folks who would do the same and are much closer to you.

Thanks!
The slew rate of the output is limited to 2V/ns, so for example a 10V rising edge input would be divided by 10x due to the probe attenuation to 1V (before 50 ohm termination), so due to the slew rate limitation would have an additional 500ps rise time (I think) (edit. around a 600ps rise, see post below). You'd have to have quite the unusual signal to be generating this kind of signal though. In my spice simulations this kind of input starts to make step responses/square waves look a little trapezoid. I don't have a high amplitidue pulse generator (or fast enough oscilloscope) to test this properly. In general, I don't know what the rise time is for more sensible signals as my oscilloscope just isn't fast enough (500MHz).

Note though that other active probes such as the Keysight N2796A 2GHz probe also limit their dynamic range at higher frequencies. This is a screenshot from their datasheet:

[lasmux]

Also if you (or anyone else) do want to verify the probes performance, I'd be happy to loan you my prototype. Especially if you have a decent VNA to check the bandwidth, and fast scope to check the rise time.
I don't mind shipping it internationally.

[lasmux]

I just ran this through my spice simulation again, and the rise time went from around 250ps to around 610ps, for a 1V rising edge or a 10V rising edge. These are just indications as my spice model isn't that similar to the actual performance of the probe. The spice version only has a bandwidth of 1.4GHz.

Edited with updated numbers.

[hpw]

Well,

to measure the slew rate, consider Bodnar Fast Pulse and a DSO may with 10Gs and RIS...

I started with an active Probe from CalTest as CT4121 (now recognized that the ground to the probe tip is missing),
have also a 2.5G HPF and now a differential AP034.

All active probe have a certain connection pins, not fully side by side as yours, but love now my single AP034

While you may simple connect an 2.54mm dual pin header as they are parallel and an additional GND pin on the side.

So easy to use 2..4 of them (as for I2S), leave them on the table along, without having the hands full of them. And turn with 2 hands on the DSO.

So my 2 cents on real measurement tasks

hp

[lasmux]

to measure the slew rate, consider Bodnar Fast Pulse and a DSO may with 10Gs and RIS...

I started with an active Probe from CalTest as CT4121 (now recognized that the ground to the probe tip is missing),
have also a 2.5G HPF and now a differential AP034.
All active probe have a certain connection pins, not fully side by side as yours, but love now my single AP034
While you may simple connect an 2.54mm dual pin header as they are parallel and an additional GND pin on the side.

That's a very fast rising edge from that pulse generator! If I manage to find a faster scope, I'll have a look at that, thanks.

[tszaboo]

Looks cool. Those socket connectors at the end, are they the same size as the Agilent active probes?
For me it's a bit strange that you choose to have the 1M to be in series with the signal, those probes have a relatively small series resistance, and the 1M is used to shunt the 1pF. Any reason for that? Not judging, I'm genuinely curious, as I haven't built high speed scope probe yet, only high gain ones for shunt resistors.